The invention relates to a release electromagnet, and more particularly, to an electromagnet including a permanent magnet which is effective to attract an armature and wherein the electromagnet produces a magnetic flux to reduce the attraction of the armature upon energization thereof.
A release electromagnet commonly referred to as an electromagnet of permanent magnet core type is used to constrain or release a movable member such as is used in an electrical shutter assembly of a camera. The electromagnet is constructed with a pair of yokes between which a permanent magnet is held in order to attract an armature thereto, and an electromagnet winding is disposed on the yoke to reduce the attraction effect upon the armature which is produced by the permanent magnet. A movable member is held attracted to the electromagnet under the magnetic influence of the permanent magnet, and when it is desired to release the movable member, the winding is energized to oppose the magnetic attraction of the permanent magnet.
Unlike a conventional electromagnet, a release electromagnet holds a movable member attracted thereto under the magnetic influence of a permanent magnet, and thus it is not necessary to maintain a holding current to hold the movable member attracted. The movable member can be released from the constraint by passing an energizing current through a winding disposed on the electromagnet for a short interval, thus achieving a substantial saving in the power dissipation. In addition, such electromagnet can be constructed in a compact manner.
Referring to FIG. 1, a prior art arrangement of a release electromagnet will be described. The electromagnet shown comprises a pair of yokes 1a, 1b formed of a soft magnetic material such as ferrite or the like in the form of square pillars. The yokes are disposed in parallel relationship with each other, and a small permanent magnet 2 formed of a rare earth metal in a square rod shape is disposed between the lower ends thereof. The end faces of the magnet which form N- and S-poles are adhesively secured to the yokes to be firmly held therebetween, thus forming a permanent magnet assembly 1A. The upper end faces of the yokes 1a, 1b represent N- and S-poles, which are effective to attract an armature 3 which represents a movable member. A winding 4 is disposed on yoke 1a, thus forming an electromagnet.
A spring 5 is anchored to the armature 3 and tends to move it away from the electromagnet. When the armature 3 is to be maintained attracted, a mechanism, not shown, is used to move the armature 3 against the resilience of spring 5 into a region in which the magnetic flux of the assembly 1A is effective to attract it. When the armature is to be released, the winding 4 is energized to counteract the attractive force of the magnet assembly 1A, allowing the armature to be moved away from the end faces 1c, 1d under the resilience of the spring 5. To achieve such release, it is necessary to pass a current through the winding 4 which is sufficient to produce a magnetic flux counteracting that from the magnet 2. It will be appreciated that the required flux which must be produced by the winding 4 in order to achieve release of the armature will amount to a substantial value, which means a poor release or separation efficiency.
FIG. 2 shows a modification which is proposed to overcome such difficulty. Specifically, the arrangement of FIG. 2 includes a bypass 1e which connects the lower ends of the yokes 1a, 1b so that the flux produced by the winding 4 can pass through the bypass to thereby improve the armature separation efficiency. However, the flux from the permanent magnet 2 will then follow two paths A and B, as shown in FIG. 2. Since a proportion of the flux is diverted to the path B, the remaining flux contained in the path A and thus effective for the purpose of attracting the armature 3, will be reduced. As a consequence, in order to increase the magnitude of the flux which in path A, the cross-sectional area d of the bypass 1 will have to be reduced so as to suppress the diversion of the flux from magnet 2. However, this tends to cause a magnetic saturation thereof upon energization of the winding 4 since the bypass 1e forms part of the path for the flux produced by the winding. Thus the alternative shown in FIG. 2 suffers from another disadvantage when it overcomes the first mentioned difficulty.